This invention relates to an epoxy molding compound formulation which offers a high degree of reliability and package cracking resistance for surface mount integrated circuits ("IC") packages. Traditionally, integrated circuits ("IC") packages have been encapsulated with a standard epoxy molding compound formulation. Attachment to a circuit board involves inserting the package pins through holes in the circuit board and soldering the protruding pins at very high temperatures (215.degree.-260.degree. C.). The circuit board limits the flow of heat to the encapsulated device; therefore, the package experiences little thermal soldering stress and has a high degree of reliability with respect to package cracking.
Currently, the trend in IC packaging is towards the surface mount type. These packages are not inserted through holes in the board, but rather are mounted directly to the board. These packages are typically thinner than conventional packages; moreover, this surface mount technology allows for packages to be soldered to both sides of a circuit board, thus doubling available space. In this soldering process, the whole package and the circuit board are exposed to the high temperature. This exposure increases the thermal soldering stress experienced by the package; the thinness of these packages in addition to the high temperature they experience greatly increases the probability of package cracking. Any moisture absorbed by the plastic encapsulant exacerbates the package cracking problem. The development of an epoxy molding compound formulation that prevents this solder-induced package cracking, as well as provides the other properties expected from the plastic encapsulant (low stress, good moldability, low ionic, etc.) is needed.
Japanese Patent Application No. 01-276653 discloses a surface mount epoxy encapsulant which contains a multifunctional epoxy, a multifunctional hardener and a liquid silicone rubber.
Japanese Patent Application No. 02-069514 discloses a multifunctional epoxy used in combination with silica particles that are required to be below 3 .mu.m in size.
Japanese Patent Application No. 63-226951 discloses a block-copolymer of novolak-organopolysiloxane, a multifunctional epoxy, a multifunctional hardener and silica.